Implants: silver and diamonds battle germs and abrasion

Interview with Prof. Bernd Stritzker, Institute of Physics, Chair of Experimental Physics IV, University of Augsburg

Almost five percent of all patients who receive a joint replacement need to go under the knife again after a short time. Bacteria that entered the body during implantation or strong surface abrasion have caused inflammation that leads to a loosening of the implant in these patients. A "precious" coating for implant materials could prevent this in the future.

In this interview with COMPAMED.de, Prof. Bernd Stritzker explains how a diamond-like coating with silver atoms is meant to reduce implant failure and how a microfluidic chip partially replaces animal tests during the development.

Prof. Stritzker, what does DLC modification mean?

Prof. Bernd Stritzker: It stands for diamond-like carbon. This popular coating technology has been well known for about ten years. It is primarily used in mechanics because these layers are extremely hard and exhibit little abrasion. This makes them also attractive for medical applications.

Abrasion due to constant load is always an important issue with joint implants and endoprostheses. Is there knowledge about these coatings in this area?

Stritzker: DLC coatings are already being used in this area. They show that abrasion is also minor in this case. Real diamonds are extremely hard due to their diamond lattice, that being the tetrahedral coordination of their atoms. Each individual carbon atom is surrounded by four other carbon atoms. Many areas in the DLC layers exhibit this diamond lattice. However, there are areas in between that are somewhat more disorganized, which is where the term "diamond-like" comes in.

You have added silver atoms to these coatings. What properties does this give an implant?

Stritzker: The silver content kills bacteria that are on the implant or have entered the surgical wound. At the moment, you try to prevent inflammation after implant surgery by administering high doses of antibiotics, which does not always work, however.

The silver coating works more precisely and locally: in the body, the implant is located in a watery environment. The silver atoms produce ions in this liquid, which kills the bacteria. However, the silver initially also kills somatic cells in the surrounding tissue. Yet we only deposit it on the surface, at about the upper 50 nm (nanometers). This is why it is used up after about a day and cells are able to grow on the implant. Subsequently no bacteria that entered the body during the implant surgery remain.

How is this coating produced?

Stritzker: The implant is immersed into an organic solution. This solution is dried in layers and then compressed with an ion beam. In doing so, the organic material is being destroyed. Hydrogen, oxygen, and other atoms evaporate while carbon remains on the surface. The energy of the ion beam generates the DLC. We do not add silver nanoparticles to the solution until the end, so they are only embedded in the upper layers.

For which implant materials can this process be used?

Stritzker: We have successfully coated both metallic materials such as titanium substrate for example and ceramic substrates.

You have tested the adhesion of bacterial cells and somatic cells with special microfluidic chips. How should we envision this?

Stritzker: By using interdigital transducers, we generate sound waves in this chip, which spread out in the liquid. These waves exert pressure on bacteria or somatic cells that adhere to the surface. When you increase the pressure, you can see after which amount of time the cells are washed away and what forces make them adhere to the surface. Overall, we hope to eliminate several animal tests with the chips by researching the cell adhesion using these physical tests. We have studied the adhesion together with our medical colleagues at the TU Munich and the University Medical Center Mannheim.

How far along is the development of a production-ready process?

Stritzker: Right now, we use so-called plasma immersion ion implantation. This is already a very cost-effective, efficient technology where any three-dimensional objects can obtain a homogeneous surface coating. So far, we actually only used flat substrates. Our industry partner, Aesculap AG, is now interested in testing the process for practical use. This is why we currently coat realistic ceramic implants since ceramic-on-ceramic bearings for knee and hip joints are presently deemed optimal by physicians. The coated devices are subsequently also directly subjected to a constant load in mechanical testing laboratories. Abrasion and forces that act on the surface are being measured here. The results already look very promising at the moment.